EEPROM semiconductor device and method of fabricating the same

ABSTRACT

There is provided an EEPROM semiconductor device including (a) a plurality of field insulating films each extending perpendicularly to word lines, (b) a plurality of memory cells arranged in a matrix, each memory cell having a floating gate, a control gate formed on the floating gate and doubling as a word line, and source and drain regions located at either sides of the control gate, (c) a common source line extending in parallel with the word lines and connecting source regions of the memory cells with each other, and (d) a first bit line extending perpendicularly to the word lines and connecting drain regions of the memory cells with each other. The above-mentioned EEPROM semiconductor device makes it possible to form CMOS logic circuit together with a non-volatile memory on a common semiconductor substrate without increasing fabrication steps, and also makes it possible for the non-volatile memory to write data thereinto and read data therefrom at a higher rate without an increase in a cell size.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor device, and more particularlyto a semiconductor device including an electrically erasableprogrammable read only memory having a two-gate structure of a floatinggate and a control gate deposited on the floating gate.

2. Description of the Related Art

An electrically erasable programmable read only memory (hereinafter,referred to simply as "EEPROM") generally includes, as a memory cell,MISFET memory transistor having a two-gate structure of a floating gateand a control gate formed on the floating gate. Data is written into oreliminated from the two-gate type EEPROM by introducing electric chargesinto or discharging electric charges from a floating gate.

For instance, data is written into the two-gate type EEPROM byintroducing channel hot electrons, generated in drain regions, into afloating gate, whereas data is eliminated from EEPROM by introducingelectrons into a source, for instance, by virtue of Fowler-Nordheimtunneling.

A conventional method of fabricating a two-gate type memory cell arrayis explained hereinbelow with reference to FIGS. 1, 2 and 3A to 3D,wherein FIG. 1 is a plan view of a conventional two-gate type memorycell array, FIG. 2 is a plan view illustrating the memory cell arraybeing fabricated, and FIGS. 3A to 3D are cross-sectional views of thememory cell array taken along the line III--III in FIG. 1, showingrespective steps of a method of fabricating the memory cell array.

As illustrated in FIG. 3A, a p-type well 2 is formed in a p-typesemiconductor substrate 1 in a region where a memory cell array is to beformed. Then, a plurality of field insulating films 3 is formed in theform of islands by selective oxidation. The field insulating films 3 arenot illustrated in FIG. 3A, but are illustrated in FIG. 2.

Then, a first gate insulating film 4 is formed all over the p-type well2, and a first polysilicon layer 5a is formed all over the first gateinsulating film 4 for forming a floating gate. Then, impurities such asphosphorus (P) are doped into the first polysilicon layer 5a by thermaldiffusion or ion-implantation to thereby lower a resistance of the firstpolysilicon layer 5a. Then, as illustrated in FIG. 2, the firstpolysilicon layer 5a is patterned into a plurality of layers 5a inparallel with each other so that the layers 5a extend perpendicularly toword lines which will be formed later, in order to define a widththereof in a direction of a channel width of a floating gate.

Then, a second gate insulating film 6 is formed all over the product,and a second polysilicon layer 7a is formed over the second gateinsulating film 6. Then, as illustrated in FIG. 3A, a patternedphotoresist film 18a is formed on the second polysilicon layer 7a byphotolithography and dry etching. The photoresist film 18a has a patternfor forming word lines.

Then, as illustrated in FIG. 3B, the second and first polysilicon layers7a and 5a are patterned with the patterned photoresist film 18a beingused as a mask, to thereby form control gates 7 and floating gates 5.After removal of the photoresist film 18a, impurities such as arsenic(As) are ion-implanted into the product with the deposited gates 5 and 7and the field insulating films 3 being used as a mask, to thereby formdrain regions 8a and source regions 8b.

Then, as illustrated in FIG. 3C, sidewall spacers 9 are formed around asidewall of the deposited gates 5 and 7 of each of memory cells in orderto cause MOS transistors located outside memory cell array regions tohave a LDD-structure. Thereafter, a first interlayer insulating film 10is deposited all over the product. The first interlayer insulating film10 is composed of boron phospho silicate glass (BPSG), and has athickness in the range of 6000 to 8000 angstroms.

Then, there is formed a photoresist film 18e having a hole above thedrain region 8a. Then, the first interlayer insulating film 10 is etchedwith the photoresist film 18e being used as a mask, to thereby form acontact hole 11 leading to the drain region 8a.

After removal of the photoresist film 18e, aluminum alloy is depositedby sputtering by a thickness in the range of 4000 to 6000 angstroms.Then, the aluminum alloy is patterned by photolithography and dryetching to thereby form bit lines 12 extending perpendicularly to theword lines. Then, the product is entirely covered with a passivationfilm 16 composed of PSG and having a thickness of about 5000 angstroms.Thus, there is completed a memory cell array.

While the above-mentioned method is being carried out, a region 3a (ahatched region in FIG. 2) which is sandwiched between the fieldinsulating films 3 and will become a source region is exposed to etchingtwice, namely, when the first polysilicon layer 5a is patterned and whenthe second polysilicon layer 7a is patterned. When the first polysiliconlayer 5a is patterned, the region 3a is covered merely with the thinfirst gate insulating film 4 after the first polysilicon layer 5a hasbeen etched. Hence, the first gate insulating film 4 is first removed,and then, the p-type semiconductor substrate 1 is undesirably etched. Inaddition, when the second polysilicon layer 7a is patterned, the region3a is covered merely with the thin second gate insulating film 6 afterthe second polysilicon layer 7a has been etched. Hence, the p-typesemiconductor substrate 1 is undesirably further etched.

As a result, as illustrated in FIG. 4 which is a cross-sectional viewtaken long the line IV--IV in FIG. 1, there is formed an undesirablerecess 19 at a surface of the semiconductor substrate 1. The undesirablerecess 19 causes junction leakage therein, which a problem thatdata-writing and data-eliminating properties are deteriorated.

If a diffusion layer had a depth shallower than a depth of the recess19, there is formed a breakage in a source region at the recess 19,since impurities are not ion-implanted into an inner sidewall of therecess 19. This causes a reduction in a fabrication yield.

The above-mentioned problem can be solved by a semiconductor devicestructure as suggested in Japanese Unexamined Patent Publications Nos.3-52267 and 3-126266, for instance. Hereinafter is explained thesuggested structure with reference to FIGS. 5, 6, 7 and 8A to 8D,wherein FIG. 5 is a plan view of the suggested memory cell array, FIG. 6is a cross-sectional view taken along the line VI--VI in FIG. 5, FIG. 7is a cross-sectional view taken along the line VII--VII in FIG. 5, andFIGS. 8A to 8D are cross-sectional views taken along the line VI--VI inFIG. 5, showing respective steps of a method of fabricating thesuggested memory cell array.

The suggested memory cell array is characterized by that a plurality ofthe field insulating films 3 extend perpendicularly to the word lines 7,and that the common source line 17a connecting the source regions 8b toeach other in a direction in which the word lines 7 extend is formed toextend perpendicularly to the field insulating films 3. Hereinafter isexplained a method of fabricating the suggested memory cell array, withreference to FIGS. 8A to 8D.

As illustrated in FIG. 8A, a p-type well 2 is formed in a p-typesemiconductor substrate 1 by introducing p-type impurities into thesemiconductor substrate 1 and thermally diffusing the p-type impuritiesin the semiconductor substrate 1. Then, a plurality of field insulatingfilms 3 are formed on a principal surface of the p-type well 2 byselective oxidation so that the field insulating films 3 extend inparallel with one another, but perpendicularly to word lines which willbe formed later. The field insulating films 3 are not illustrated inFIG. 8A, but only illustrated in FIG. 5.

Then, a first gate insulating film 4 and then a first polysilicon layer5a are formed all over the product. Then, impurities such as phosphorus(P) are ion-implanted into the first polysilicon layer 5a to therebylower a resistance of the first polysilicon layer 5a. Then, asillustrated in FIG. 2, the first polysilicon layer 5a is patterned intoa plurality of layers 5a in parallel with each other in order to definea width thereof in a direction of a channel width of a floating gate.When the first polysilicon layer 5a is patterned, the thick fieldinsulating films 3 exist below a region where the first polysiliconlayer 5a is etched, which ensures that the substrate 1 is not etched,and hence a recess such as the recess 19 illustrated in FIG. 4 is notformed.

Then, a second gate insulating film 6 is formed all over the product,and a second polysilicon layer 7a is formed over the second gateinsulating film 6. Then, impurities such as phosphorus (P) areion-implanted into the second polysilicon layer 7a to thereby lower aresistance thereof Then, as illustrated in FIG. 8A, a patternedphotoresist film 18a is formed on the second polysilicon layer 7a byphotolithography and dry etching. The photoresist film 18a has a patternfor forming word lines.

Then, as illustrated in FIG. 8B, the second and first polysilicon layers7a and 5a are patterned by etching with the patterned photoresist film18a being used as a mask, to thereby form control gates 7 and floatinggates 5. After removal of the photoresist film 18a , n-type impuritiesare ion-implanted into the product with the deposited gates 5 and 7 andthe field insulating films 3 being used as a mask, to thereby form drainregions 8a and source regions 8b.

Then, as illustrated in FIG. 8C, sidewall spacers 9 are formed around asidewall of the deposited gates 5 and 7 of each of memory cells.Thereafter, a first interlayer insulating film 10 is deposited all overthe product by chemical vapor deposition (CVD). The first interlayerinsulating film 10 is composed of silicon dioxide. Then, the firstinterlayer insulating film 10 is etched in selected regions to therebyform contact holes C1 reaching a surface of the source regions 8b andcontact holes C2 reaching a surface of the drain regions 8a.

Then, as illustrated in FIG. 8D, an electrically conductive layercomposed of polysilicon is formed all over the product, and thenpatterned to thereby form a common source line 17a and an extended bitline 17b. The common source line 17a connects the source regions 8b in adirection in which the word lines extend. The extended bit line 17bmakes electrical contact with the drain region 8a through the contacthole C2, and covers a portion of the first interlayer insulating film 10around the contact hole C2 therewith. The electrically conductive layerfrom which the common source line 17a and the extended bit line 17b areformed may be composed of refractory metal, silicide thereof, orpolycide thereof, as well as polysilicon.

Then, a second interlayer insulating film 13 composed of BPSG isdeposited all over the product. Thereafter, a photoresist film 18c isformed, and then, patterned by photolithography and dry etching so as tohave an opening above the extended bit line 17b. Then, the secondinterlayer insulating film 13 is etched with the patterned photoresistfilm 18c being used as a mask, to thereby form through-holes 14 reachingthe extended bit line 17b.

After removal of the photoresist film 18c, aluminum alloy is depositedby sputtering. Then, the aluminum alloy is patterned by photolithographyand dry etching to thereby form bit lines 12 (see FIGS. 5, 6 and 7)extending perpendicularly to the word lines. Then, the product isentirely covered with a passivation film 16 (see FIGS. 4, 6 and 7)composed of PSG. Thus, there is completed a non-volatile semiconductormemory device as illustrated in FIGS. 4 to 7.

In accordance with the above-mentioned method, when the firstpolysilicon layer 5a is etched, the thick field insulating films 3 existbelow a region to be etched. When the second and first polysiliconlayers 7a and 5a are patterned to thereby form the control gate 7 andthe floating gate 5, a region where only a single polysilicon layer isetched is a region located above the field insulating regions 3. Hence,the above-mentioned undesirable recess 19 caused by etching apolysilicon layer is not formed. Accordingly, there is solved a problemthat junction leakage junction leakage occurs due to the recess, andresultingly data-writing and data-eliminating properties aredeteriorated, and that a fabrication yield due to the breakage in asource region is reduced.

A semiconductor device including a high-rate CMOS logic circuit isgenerally designed to have two or more wiring layers. When anon-volatile memory is formed on a common semiconductor substrate onwhich a high-rate CMOS logic circuit is also formed, it is required thatan increase in the number of additional fabrication steps is avoided andthat the non-volatile memory is small in size, in order to reducefabrication costs and integrate the device in a higher density.

In the conventional method having been explained with reference to FIGS.5, 6, 7 and 8A to 8D, the common source line is formed of theelectrically conductive layer composed of electrically conductivematerial such as polysilicon, after the contact hole has been formed.Hence, the above-mentioned conventional method has a problem that thenumber of additional fabrication steps is increased relative to thenumber of steps for fabricating CMOS logic circuit having two or morewiring layers, and hence, fabrication costs are also increased.

In addition, since the common source line is formed of an electricallyconductive layer composed of polysilicon, the common source lineunavoidably has high resistivity, which causes problems thatdata-writing and data-eliminating properties of a non-volatile memoryare deteriorated, and that a speed at which a memory cell reads out datais reduced.

The common source line may be designed to have a smaller resistance byincreasing an area of the electrically conductive layer and/or forming abacking wiring layer composed of aluminum. However, this makes itdifficult to reduce a size of a memory cell, and reduce fabricationcosts per a chip.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electricallyerasable programmable read only memory which is capable of being formedcommonly on a semiconductor substrate on which a high-rate CMOSsemiconductor device is also formed, without an increase in the numberof additional fabrication steps, and also capable of writing datathereinto and reading data therefrom at a high rate without an increasein a cell size.

The above-mentioned object can be accomplished by presenting anon-volatile memory including memory cells having a floating gate and acontrol gate doubling as a word line, field insulating films eachextending perpendicularly to word lines to thereby electrically insulatethe memory cells from one another, a common source line extending inparallel to the word lines to thereby connect source regions of thememory cells to one another, and a bit line extending perpendicularly tothe word lines to thereby connect drain regions of the memory cells toone another. The common source line may be formed of a first metalwiring layer, and the bit line may be formed of a second metal wiringlayer.

Specifically, in one aspect of the present invention, there is providedan EEPROM semiconductor device including (a) a plurality of fieldinsulating films each extending perpendicularly to word lines, (b) aplurality of memory cells arranged in a matrix, each memory cellincluding a floating gate, a control gate formed on the floating gateand doubling as a word line, and source and drain regions located ateither sides of the control gate, (c) a common source line extending inparallel with the word lines and connecting source regions of the memorycells with each other, and (d) a first bit line extendingperpendicularly to the word lines and connecting drain regions of thememory cells with each other.

The common source line may be constituted of a first metal wiring layer,which is preferably composed of aluminum. The bit line may beconstituted of a second metal wiring layer, which is preferably composedof aluminum.

The EEPROM semiconductor device may further include a plurality ofsecond bit lines formed above the drain regions of the memory cells, inwhich case, it is preferable that the first bit line connects the secondbit lines with one another.

It is preferable that both the second bit lines and the common sourceline are constituted of a first metal wiring layer, which is preferablycomposed of aluminum.

The EEPROM semiconductor device may further include CMOS logic circuitincluding both the common source line and the first bit line, and formedon a common semiconductor substrate.

There is further provided an EEPROM semiconductor device including (a) aplurality of field insulating films each extending perpendicularly toword lines, (b) a plurality of memory cells arranged in a matrix, eachmemory cell including a floating gate, a control gate formed on thefloating gate and doubling as a word line; and source and drain regionslocated at either sides of the control gate, (c) a first bit lineextending perpendicularly to the word lines and connecting drain regionsof the memory cells with each other, and (d) a first common source lineextending in parallel with the word lines and connecting source regionsof the memory cells with each other.

The EEPROM semiconductor device may further include a plurality ofsecond common source lines formed above the source regions of the memorycells, in which case, the first common source line preferably connectsthe second common source lines with one another.

It is preferable that both the second common source lines and the bitline are constituted of a first metal wiring layer, which is preferablycomposed of aluminum.

The EEPROM semiconductor device may further include backing wiringlayers each of which is connected to the word lines at every certainnumber of bits, in which case, it is preferable that both the backingwiring layers and the first common source lines are constituted of asecond metal wiring layer.

In another aspect, there is provided a method of fabricating an EEPROMsemiconductor device, including the steps of (a) forming a plurality offield insulating films in parallel on a semiconductor substrate, (b)forming a first gate insulating film in each of active regions, (c)forming a plurality of first polysilicon layers in parallel with oneanother perpendicularly to word lines, (d) forming a second gateinsulating film and a second polysilicon layer all over the productresulting from the step (c), (e) patterning the second polysiliconlayer, the second gate insulating film, and the first polysilicon layerto thereby form a control gate and a floating gate, (f) forming drainand source regions, (g) forming a first interlayer insulating layer allover the product resulting from the step (f), (h) forming a first metalwiring layer which is patterned so as to form both a common source lineextending in parallel with the word lines and connecting source regionsto one another, and an extended bit line connecting the drain region toa bit line, (i) forming a second interlayer insulating layer all overthe product resulting from the step (h), and (j) forming a second metalwiring layer which is patterned so as to form a bit line connecting thedrain regions to one another.

The second gate insulating film may have a three-layered structure ofoxide/nitride/oxide films.

There is further provided a method of fabricating an EEPROMsemiconductor device, including the steps of (a) forming a plurality offield insulating films in parallel on a semiconductor substrate, (b)forming a first gate insulating film in each of active regions, (c)forming a plurality of first polysilicon layers in parallel with oneanother perpendicularly to word lines, (d) forming a second gateinsulating film and a second polysilicon layer all over the productresulting from the step (c), (e) patterning the second polysiliconlayer, the second gate insulating film, and the first polysilicon layerto thereby form a control gate and a floating gate, (f) forming drainand source regions, (g) forming a first interlayer insulating layer allover the product resulting from the step (f), (h) forming a first metalwiring layer which is patterned so as to form both a bit line extendingalmost in parallel with the field insulating films and connecting drainregions to one another, and an extended common source line connectingthe source region to a later mentioned common source line, (i) forming asecond interlayer insulating layer all over the product resulting fromthe step (h), and (j) forming a second metal wiring layer which ispatterned so as to form a common source line connecting the sourceregions to one another.

The method may further include the step of forming backing wiring layersconnecting to the control gate at a certain interval, in which case, thebacking wiring layers are preferably constituted of the second metalwiring layer.

The above and other objects and advantageous features of the presentinvention will be made apparent from the following description made withreference to the accompanying drawings, in which like referencecharacters designate the same or similar parts throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a conventional memory cell array.

FIG. 2 is a plan view of the conventional memory cell array illustratedin FIG. 1, being fabricated.

FIGS. 3A to 3D are cross-sectional views taken along the line III--IIIin FIG. 1, illustrating respective steps of a method of fabricating thememory cell array illustrated in FIG. 1.

FIG. 4 is a cross-sectional view taken along the line IV--IV in FIG. 1.

FIG. 5 is a plan view of another conventional memory cell array.

FIG. 6 is a cross-sectional view taken along the line VI--VI in FIG. 5.

FIG. 7 is a cross-sectional view taken along the line VII--VII in FIG.5.

FIGS. 8A to 8D are cross-sectional views taken along the line VI--VI inFIG. 5, illustrating respective steps of a method of fabricating thememory cell array illustrated in FIG. 5.

FIG. 9 is a plan view of a memory cell array in accordance with thefirst embodiment of the present invention.

FIG. 10 is a plan view of the memory cell array illustrated in FIG. 9,being fabricated.

FIGS. 11A to 11E are cross-sectional views taken along the line 11E--11Ein FIG. 9, illustrating respective steps of a method of fabricating thememory cell array illustrated in FIG. 9.

FIG. 12 is a plan view of a memory cell array in accordance with thesecond embodiment of the present invention.

FIG. 13 is a cross-sectional view taken along the line XIII--XIII inFIG. 12.

FIG. 14 is a cross-sectional view taken along the line XIV--XIV in FIG.12.

FIG. 15 is a cross-sectional view taken along the line XV--XV in FIG.12.

FIGS. 16A and 16B are cross-sectional views taken along the lineXIII--XIII in FIG. 12, illustrating respective steps of a method offabricating the memory cell array illustrated in FIG. 12.

FIG. 17 is a plan view of a memory cell array in accordance with thethird embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 9 illustrates a memory cell array in accordance with the firstembodiment. As illustrated in FIG. 9, a plurality of field insulatingfilms 3 is formed in parallel perpendicularly to word lines. Controlgates 7 doubling as word lines extend perpendicularly to the fieldinsulating films 3. Floating gates 5 are formed on channel regionslocated below the control gates 7. That is, the control gates 7 aredeposited on the floating gates 5. Drain regions 8a and source regions8b are formed in a semiconductor substrate at either sides of thedeposited gates 7 and 5.

The source regions 8b are connected to each other via contact holes 11through a common source line 12a extending in parallel with the wordlines and composed of a first aluminum wiring layer. The drain regions8a are connected to extended bit lines 12b composed of the firstaluminum wiring layer via the contact hole 11, and are connected to oneanother through a bit line 15a composed of a second aluminum wiringlayer in a direction perpendicular to the word lines.

A method of fabricating the memory cell array in accordance with thefirst embodiment is explained hereinbelow with reference to FIGS. 11A to11E.

As illustrated in FIG. 11A, a p-type semiconductor substrate 1 ision-implanted at about 100 KeV with doses of about 1×10¹³ atoms/cm² withp-type impurities such as boron (B), followed by annealing at about1000°C. Thus, there is formed a p-type well 2 in the p-typesemiconductor substrate 1 in a region where a memory cell array is to beformed.

Then, a plurality of field insulating films 3 composed of silicondioxide are formed in parallel by selective oxidation. The fieldinsulating films 3 extend perpendicularly to word lines which will beformed later, and have a thickness in the range of 4000 to 8000angstroms. The field insulating films 3 are not illustrated in FIG. 11A,but only illustrated in FIG. 10.

Then, a substrate surface of active regions are thermally oxidized at atemperature in the range of 700 to 850 degrees centigrade to therebyform a first gate insulating film 4 which will make a gate oxide film ofmemory cells. The thus formed first gate insulating film 4 has athickness of about 100 angstroms.

Then, a first polysilicon layer 5a is formed all over the first gateinsulating film 4 by a thickness in the range of about 1500 to about2500 angstroms by reduced pressure CVD. The first polysilicon layer 5awill make a floating gate. Then, n-type impurities such as phosphorus(P) are doped into the first polysilicon layer 5a by thermal diffusionor ion-implantation to thereby lower a resistance of the firstpolysilicon layer 5a.

Then, as illustrated in FIG. 10, the first polysilicon layer 5a ispatterned by photolithography and dry etching into a plurality of layers5a in parallel with each other so that the layers 5a extendperpendicularly to word lines which will be formed later, in order todefine a width thereof in a direction of a channel width of a floatinggate.

When the first polysilicon layer 5a is patterned, the thick fieldinsulating films 3 exist below a region where the first polysiliconlayer 5a is etched, which ensures that the substrate 1 is not etched inan etching step for forming a gate electrode, and hence a recess such asthe recess 19 illustrated in FIG. 4 is not formed.

Then, a second gate insulating film 6 having a thickness in the range ofabout 200 to about 300 angstroms is formed all over the product bythermal oxidation or CVD. The second gate insulating film 6 may bedesigned to have a three-layered structure of oxide/nitride/oxide films,which called ONO film.

The second gate insulating film 6 formed outside a region where memorycell array is to be formed is removed by wet or dry etching, using acidsuch as hydrofluoric acid. Thereafter, a second polysilicon layer 7a isformed all over the second gate insulating film 6 by reduced pressureCVD. The second polysilicon layer 7a will make a control gate and a gateelectrode of peripheral transistors. Then, n-type impurities such asphosphorus (P) are introduced into the second polysilicon layer 7a bythermal diffusion or ion-implantation to thereby lower a resistancethereof. On the second polysilicon layer 7a may be formed a filmcomposed of silicide of refractory metal such as W, Ti and Mo to therebyform a polycide structure film.

Then, as illustrated in FIG. 11A, a patterned photoresist film 18a isformed on the second polysilicon layer 7a by photolithography and dryetching. The photoresist film 18a has a pattern for forming controlgates.

Then, as illustrated in FIG. 11B, the second polysilicon layer 7a, thesecond gate insulating film 6 and the first polysilicon layer 5a arepatterned by reactive ion etching (RIE) with the patterned photoresistfilm 18a being used as a mask, to thereby form control gates 7 andfloating gates 5 in self-align fashion.

After removal of the photoresist film 18a, the product is ion-implantedat about 50 to 70 KeV with doses of about 1×10¹⁵ atoms/cm² with n-typeimpurities such as arsenic (As) with the deposited gates 5 and 7 and thefield insulating films 3 being used as a mask, to thereby form drainregions 8a and source regions 8b.

Then, as illustrated in FIG. 11C, sidewall spacers 9 are formed around asidewall of the deposited gates 5 and 7 of each of memory cells in orderto cause CMOS transistors located outside memory cell array regions tohave a LDD-structure. Thereafter, a first interlayer insulating film 10is deposited all over the product by chemical vapor deposition (CVD).The first interlayer insulating film 10 is composed of BPSG and has athickness in the range of 6000 to 8000 angstroms. Then, there is formeda photoresist film 18b by photolithography and dry etching. Thephotoresist film 18b has openings above the source regions 8b and thedrain regions 8a . Then, the first interlayer insulating film 10 isetched by RIE in selected regions with the photoresist film 18b beingused as a mask, to thereby form contact holes 11 reaching all the sourceand drain regions 8b and 8a of the memory cells.

Then, as illustrated in FIG. 11D, aluminum alloy is deposited over theproduct by sputtering by a thickness in the range of about 4000 to about6000 angstroms. The thus deposited aluminum alloy is patterned tothereby form a common source line 12a and an extended bit line 12b bothas a first aluminum wiring layer. The common source line 12a extends inparallel with the word lines, and connects the source regions 8b locatedin a direction in which the word lines extend, to one another. Theextended bit line 12b is a junction through which the drain regions 8amake electrical contact with a bit line.

Then, a second interlayer insulating film 13 composed of BPSG isdeposited all over the product by CVD. The second interlayer insulatingfilm 13 has a thickness in the range of about 4000 to about 5000angstroms. Thereafter, a photoresist film 18c is formed, and then,patterned by photolithography and dry etching so as to have an openingabove the drain regions 8a. Then, the second interlayer insulating film13 is etched by RIE with the patterned photoresist film 18c being usedas a mask, to thereby form through-holes 14 reaching the extended bitline 12b.

After removal of the photoresist film 18c, as illustrated in FIG. 11E,an aluminum alloy film having a thickness in the range of about 4000 toabout 6000 angstroms, as a second aluminum wiring layer, is deposited bysputtering over the product. Then, the aluminum alloy film is patternedby photolithography and dry etching to thereby form bit lines 15a inparallel with the field insulating film 3. The bit lines 15a connect thedrain regions 8a located adjacent to the field insulating film 3, to oneanother.

In CMOS logic products where a memory cell is formed on a commonsubstrate, wirings are also made in CMOS logic circuit in first andsecond metal wiring layers in a memory cell array region. If a contacthole or a through-hole were filled with metal such as tungsten (W) inCMOS logic circuit, wirings can be made in the same manner also in amemory cell array.

Then, the product is entirely covered with a passivation film 16composed of PSG. Thus, there is completed the memory cell array inaccordance with the first embodiment.

Second Embodiment

FIGS. 12 to 15 illustrate a memory cell array in accordance with thesecond embodiment.

The second embodiment is different from the first embodiment in that thebit line 12c is constituted of the first aluminum wiring layer, and thecommon source line 15b is constituted of the second aluminum wiringlayer. In the second embodiment, the drain regions 8a arranged inparallel with the field insulating films 3 are connected to one anotherthrough the bit line 12c constituted of the first aluminum wiring layerand extending almost in parallel with the field insulating films 3, anda common extended source line 12d constituted of the first aluminumwiring layer is formed on the source regions 8b. The source regions 8barranged in parallel with the word lines are connected commonly to thecommon source line 15b via the common extended source line 12d. Thecommon source line 15b is constituted of the second aluminum wiringlayer, and extends in parallel with the word lines.

A method of fabricating the memory cell array in accordance with thesecond embodiment is explained hereinbelow with reference to FIGS. 16Aand 16B.

The method of fabricating the memory cell array in accordance with thesecond embodiment has the same fabrication steps from the first step tothe step illustrated in FIG. 11C as those in the method of fabricatingthe memory cell array in accordance with the first embodiment.

As illustrated in FIG. 11C or FIG. 16A, there are formed the contactholes 11 reaching all the drain regions 8a and source regions 8b formedin the memory cell array.

Then, as illustrated in FIG. 16B, aluminum alloy is deposited over theproduct by sputtering by a thickness in the range of about 4000 to about6000 angstroms. The thus deposited aluminum alloy is patterned tothereby form a bit line 12c and a common extended source line 12d bothas a first aluminum wiring layer. The bit line 12c extends almost inparallel with the field insulating films 3, and connects the drainregions 8a located in parallel with the field insulating films 3, to oneanother. The common extended source line 12d is a junction through whichthe source regions 8b make electrical contact with the common sourceline 15b.

Then, a second interlayer insulating film 13 composed of BPSG isdeposited all over the product by CVD. The second interlayer insulatingfilm 13 has a thickness in the range of about 4000 to about 5000angstroms. Thereafter, a photoresist film 18d is formed, and then,patterned by photolithography and dry etching so as to have an openingabove the source regions 8b. Then, the second interlayer insulating film13 is etched by RIE with the patterned photoresist film 18d being usedas a mask, to thereby form through-holes 14 reaching the common extendedsource line 12d.

After removal of the photoresist film 18d, as illustrated in FIG. 16B,an aluminum alloy film having a thickness in the range of about 4000 toabout 6000 angstroms, as a second aluminum wiring layer, is deposited bysputtering over the product. Then, the aluminum alloy film is patternedby photolithography and dry etching to thereby form a common source line15b in parallel with the word lines. The common source line 15b connectsthe source regions 8b located in parallel with the word lines, to oneanother.

Then, the product is entirely covered with a passivation film 16composed of PSG. Thus, there is completed the memory cell array inaccordance with the second embodiment as illustrated in FIG. 13.

In the above-mentioned second embodiment, a wiring layer constituted ofthe second aluminum wiring layer is only the common source line 15b.Hence, it is possible for the common source line 15b to have a greaterwidth than a width of a common source line in the first embodiment,which ensures a further reduction in a resistance of the common sourceline 15b, resulting in that the memory cell could operate at a higherrate.

Third Embodiment

FIG. 17 is a plan view illustrating a memory cell array in accordancewith the third embodiment. Parts or elements corresponding to those ofthe memory cell array in accordance with the second embodimentillustrated in FIG. 12 have been provided with the same referencenumerals, and are not explained in detail.

The third embodiment is different from the second embodiment in thatbacking wiring layers 15c constituted of the second aluminum wiringlayer are formed above the control gates 7, and connect to the controlgates 7 at a certain interval.

In the second embodiment, since the second aluminum wiring layer isformed only into the common source line 15b, the common source line 15bwas designed to have a greater width for lowering a resistance thereof.In the third embodiment, it is possible to operate a memory cell arrayat a higher rate by lowering a resistance of word lines.

The backing wiring layers 15c illustrated in FIG. 17 are designed to beconnected to the word lines or control gates 7 via contact holes atevery 32 bits, for instance.

The word lines are generally composed of polysilicon or polycide.However, these materials have greater resistivity than other metals. Inaddition, since the word lines are so long, a great degree of RC isgenerated in the word lines, and may cause a memory cell array tooperate at a lower rate. To the contrary, in accordance with the presentembodiment, the backing wiring layers 15c lower a resistance of the wordlines, and hence, data-reading can be accomplished at a higher rate.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

The entire disclosure of Japanese Patent Application No. 9-205592 filedon Jul. 31, 1997 including specification, claims, drawings and summaryis incorporated herein by reference in its entirety.

What is claimed is:
 1. An EEPROM semiconductor device comprising:(a) aplurality of field insulating films each extending perpendicularly toword lines; (b) a plurality of memory cells arranged in a matrix, eachmemory cell including a floating gate, a control gate formed on saidfloating gate, the control gate also operating as one of said word linesthat corresponds to a row in the matrix in which said each memory cellis disposed, and having source and drain regions located at either sidesof said control gate; (c) a first bit line extending perpendicularly tosaid word lines and connecting drain regions of said memory cells witheach other, said first bit line corresponding to a first metal wiringlayer; (d) a first common source line extending in parallel with saidword lines and connecting source regions of said memory cells with eachother, said first common source line being disposed entirely above saidword lines with respect to a distance from a semiconductor substrate,said first common source line having a top portion and a bottom portionwith the top portion being wider than the bottom portion; and (e) acommon extended source line that extends downwardly from said firstcommon source line so as to connect said first common source line to asource region disposed within said semiconductor substrate, said commonextended source line having a top portion and a bottom portion with thetop portion of said common extended source line connected to the bottomportion of said first common source line, and with the top portion ofsaid common extended source line being wider than the bottom portion ofsaid common extended source line, wherein said first common source linecorresponds to a second metal wiring layer, wherein said first commonsource line has a greater width than each of said word lines to resultin a low resistance of said first common source line.
 2. The EEPROMsemiconductor device as set forth in claim 1, wherein said second metalwiring layer is composed of aluminum.
 3. An EEPROM semiconductor devicecomprising:(a) a plurality of field insulating films each extendingperpendicularly to word lines; (b) a plurality of memory cells arrangedin a matrix, each memory cell including a floating gate, a control gateformed on said floating gate, the control gate also operating as one ofsaid word lines that corresponds to a row in the matrix in which saideach memory cell is disposed, and having source and drain regionslocated at either sides of said control gate; (c) a first bit lineextending perpendicularly to said word lines and connecting drainregions of said memory cells with each other; (d) a first common sourceline extending in parallel with said word lines and connecting sourceregions of said memory cells with each other; and (e) backing wiringlayers disposed above said word lines, each of said backing wiringlayers being connected to a corresponding one of said word lines abovewhich said each backing wiring layer is disposed, only at every i numberof bits, wherein i is an integer greater than one, wherein a firstportion of each of said backing wiring layers overlaps a correspondingone of said word lines and a second portion of said each of said backingwiring layers does not overlap said corresponding one of said wordlines, and wherein said second portion is disposed closer to said firstcommon source line than said first portion.
 4. The EEPROM semiconductordevice as set forth in claim 3, wherein both said backing wiring layersand said first common source lines are constituted of a second metalwiring layer.
 5. The EEPROM semiconductor device as set forth in claim3, wherein said backing wiring layers coupled to said word lines lower aresistance of said word lines with respect to said word lines that arenot connected to said backing wiring layers.